1. Field of the Invention
The present invention relates to a system for supplying secondary air into an exhaust system of an internal combustion engine for the purpose of purifying the exhaust gas discharged from the engine.
2. Description of the Prior Art
As a method of purifying exhaust gases discharged from an internal combustion engine, it is known to introduce secondary air into the exhaust system of the engine so as to recombust harmful components such as HC and CO contained in the exhaust gases. In order to obtain an optimum result from the secondary air injection in view of the degree of exhaust gas purification as well as to protect the hardware constituting the exhaust system of the engine, it is preferable that the amount of secondary air introduced into the exhaust system is controlled so that a predetermined constant ratio is maintained between the amount of engine intake air and that of the secondary air.
Since the amount of engine intake air is directly related to the venturi vacuum, it would be easy to control the supply of secondary air depending upon the venturi vacuum by employing a flow control valve adapted to be operated by said vacuum. However, since the venturi vacuum is proportional to the square of the amount of engine intake air, a relatively complicated control system is required to effect a non-linear modification between the amount to be controlled, i.e., the amount of secondary air, and a control signal, i.e., the venturi vacuum, if a linear relation is to be maintained between these two amounts throughout the entire operational region of the engine.